Focal seizures are induced by the injection of penicillin into the hand-face area of the right motor cortex of the macaca mulatta at successive age levels from birth to young adulthood. The evolving seizure pattern in the developing brain is recorded by electrocorticography, electromyography, and clinical observations. The subcortical neuronal aggregates that are sequentially involved in the propagation of the seizure activity are investigated by the use of regional cerebral blood flow determinations with the (C14) antipyrine method, cathecholamine determinations by micro-assay methods, and depth recording from the cerebellar cortex and roof nuclei.